Nonlinear responses to an optical field are universal in nature but have been difficult to observe in the extreme ultraviolet (XUV) and soft X-ray regions owing to a lack of coherent intense light sources. High harmonic generation is a well-known nonlinear optical phenomenon and is now drawing much attention in attosecond pulse generation. For the application of high harmonics to nonlinear optics in the XUV and soft X-ray regime, optical pulses should have both large pulse energy and short pulse duration to achieve a high optical electric field. Here we show the generation of intense isolated pulses from a single harmonic (photon energy 27.9 eV) by using a sub-10-femtosecond blue laser pulse, producing a large dipole moment at the relatively low (ninth) harmonic order nonadiabatically. The XUV pulses with pulse durations of 950 attoseconds and 1.3 femtoseconds were characterized by an autocorrelation technique, based on two-photon above-threshold ionization of helium atoms. Because of the small cross-section for above-threshold ionization, such an autocorrelation measurement of XUV pulses with photon energy larger than the ionization energy of helium has not hitherto been demonstrated. The technique can be extended to the characterization of higher harmonics at shorter wavelengths.
"They are employed in conventional radio-frequency accelerators for nuclear spectroscopy measurements, in pulsed x-ray machines to create hot plasma pinch, and in the investigation of inertial confinement fusion . There has been extensive use of gas jets in studies of laser-plasma interactions, such as generation of high-harmonics , THz , extreme ultraviolet radiation , x-rays , and ion beams . Underdense plasma targets based on gas jets have been used for laser pulse compression , laser frequency up-shift , and smoothing of the laser beam intensity fluctuation . "
[Show abstract][Hide abstract] ABSTRACT: We report experimental results on the production and characterization of asymmetric and composite supersonic gas flows, created by merging independently controllable flows from multiple nozzles. We demonstrate that the spatial profiles are adjustable over a large range of parameters, including gas density, density gradient, and atomic composition. The profiles were precisely characterized using three-dimensional tomography. The creation and measurement of complex gas flows is relevant to numerous applications, ranging from laser-produced plasmas to rocket thrusters.
"uld be an attosecond pulse train . The race was on to test this spectral phase relation . As attosecond pulses typically come with low pulse energies , nonlinear - optical autocorrelation commonly applied for femtosecond pulses was not immediately possible , and still is not a viable option for all photon - energy ranges ( Tzallas et al . , 2003 ; Sekikawa et al . , 2004 ) . Instead , the solution was a temporal cross correlation of the HHG light with a moderately intense and coherently locked copy of the 800 nm HHG driver pulse in a gas medium while observing photo - electron emission ( Paul et al . , 2001 ) . In this process , the odd harmonic photons with energy E 2n+1 ionized an electron with excess"
[Show abstract][Hide abstract] ABSTRACT: In the past two decades high-harmonic generation (HHG) has become a key
process in ultra-fast science due to the extremely short time-structure of the
underlying electron dynamics being imprinted in the emitted harmonic light
bursts. After discussing the fundamental physical picture of HHG including
continuum--continuum transitions, we describe the experimental progress
rendering HHG to the unique source of attosecond pulses. The development of
bright photon sources with zeptosecond pulse duration and keV photon energy is
underway. In this article we describe several approaches pointed toward this
aim and beyond. As the main barriers for multi-keV HHG, phase-matching and
relativistic drift are discussed. Routes to overcome these problems are pointed
out as well as schemes to control the HHG process via alterations of the
driving fields. Finally, we report on how the investigation of fundamental
physical processes benefits from the continuous development of HHG sources.
Advances in Atomic, Molecular, and Optical Physics 01/2012; 61. DOI:10.1016/B978-0-12-396482-3.00004-1 · 5.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: High-order harmonic emission can be confined to the leading edge of an 800 nm driver laser pulse under moderately intense focusing conditions (7×10^14 W/cm^2) (Pfeifer et al. in Opt. Express 15:17120, 2007). Here, the experimentally observed curtailment of harmonic production on the leading edge of the driver pulse is shown to be controlled by an ionization-induced phase-matching condition. The transient plasma density inherent to the process of high-harmonic generation terminates the harmonic emission by an ultrafast loss of phase matching on the leading edge of the laser pulse. The analysis is supported by a reconstruction of the in situ intensity envelope of the driver pulse with attosecond temporal resolution, performed by measurements of the carrier-envelope phase dependence of individual half-cycle harmonic cutoffs. The method opens the way to wavelength-tunable isolated attosecond pulse generation.
Applied Physics B 11/2008; 93(2-3). DOI:10.1007/s00340-008-3187-z · 1.86 Impact Factor
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